Digital Subscriber Line (DSL) is a high-speed transmission technology that uses Unshielded Twist Pairs (UTPs) to transmit data, and comes in the following types: Asymmetrical Digital Subscriber Line (ADSL), Very-high-bit-rate Digital Subscriber Line (VDSL), ISDN-based Digital Subscriber Line (IDSL, where ISDN represents Integrated Services Digital Network), Single-pair High-bit-rate Digital Subscriber Line (SHDSL), Asymmetrical Digital Subscriber Line 2 (ADSL2), Asymmetrical Digital Subscriber Line 2plus (ADSL2plus) and Very-high-bit-rate Digital Subscriber Line 2 (VDSL2), etc.
In all sorts of xDSL technologies except IDSL and SHDSL, passband transmission is commonly used, and a Frequency Division Multiplexing (FDM) technology is utilized to make the DSL and the Plain Old Telephone Service (POTS) coexist on the same UTP, in which the DSL occupies the high frequency band and the POTS occupies the baseband part which is lower than 4 KHz, the POTS signals are separated from the DSL signals through a splitter, or the POTS signals are combined with the DSL signals through a combiner. The passband transmission-based xDSL employs a Discrete Multi-Tone (DMT) modulation technology for modulation and demodulation.
With the development of the DSL technology, the transmission rate is higher and higher, and the spectral range is wider and wider, in practical applications, there are application scenes in which many different DSL are used to build a net, thus crosstalk noise affects services increasingly. As shown in FIG. 1, a transceiver of a roadside cabinet 102 shares a bundle of cables with a transceiver of an equipment room 100 of a local Central Office (CO). A signal sent by the transmitter of the CO equipment room 100 reaches the far end after being transmitted for a length (E-side length). Due to attenuation, the signal arriving at the far end is weak, if the amplitude of the signal sent by the transmitter of the roadside cabinet 102 exceeds a specific amplitude value, such signal imposes serious crosstalk on the signal sent by the transmitter of the CO equipment room 100, thus affects performance and stability of the signal sent by the transmitter of the CO equipment room 100, or even leads to the user to loss their connection.
As shown in FIG. 2, suppose that the transmitter of the CO equipment room 100 sends ADSL signals, the transmitter of the roadside cabinet 102 sends VDSL signals, and the ASDL signal arriving at the roadside cabinet 102 is ADSL signal 200. Since the amplitude of the VDSL signal sent by the roadside cabinet 102 exceeds a specific amplitude value, the VSDL signal sent by the transmitter of the roadside cabinet 102 causes crosstalk noise 202 and crosstalk noise 204 to the ADSL signal 200, and the crosstalk noise is aliased into the band to form an aliasing noise, which affects the performance and stability of the ADSL signal 200.
In the prior art, a solution to reducing the influence of the crosstalk caused by the signals sent by the roadside cabinet 102 to the ADSL signal 200 is to reduce the power of the signals sent by the transmitter of the roadside cabinet 102, namely, to perform Downstream Power Back Off (DPBO). As shown in FIG. 3, after the process of DPBO, the signals sent by the transmitter of the roadside cabinet 102 cause less crosstalk to the ADSL signals 200, and theoretically, the crosstalk is not aliased into the band to affect the ADSL signals 200. However, since most modems available now are not much capable of anti-aliasing filtering, therefore, after the process of DPBO, the crosstalk noise generated by the signals sent by the transmitter of the roadside cabinet 102 is actually aliased into the band. As shown in FIG. 4, the aliasing noise in the band affects the performance and stability of in-band signals.